Impact of land use, fertilization and seasonal variation on the abundance and diversity of nirS-type denitrifying bacterial communities in a Mollisol in Northeast China

Seasonal variations and land use responses of nirS-type denitrifying bacterial communities were investigated in a cultivated black soil (0-20 cm) in Northeast China with different fertilization treatments [no fertilizer (NoF), chemical fertilizer (CF) and CF plus manure (CFM)] and in a neighbouring plot of natural restoration (NR) using qPCR and Illumina Miseq sequencing. The NR plot was revegetated naturally with grasses without human disturbance since 1985. In general, the abundance of nirS-type denitrifying bacteria did not change significantly between NR and NoF across all sampling times, but fertilization increased the abundance of the nirS gene, except for samples collected in October. Aromatoleum, Rhodanobacter, Cupriavidus, Bradyrhizobium, unclassified genus of Proteobacteria, Magnetospirillum and unclassified genus of Rhodocyclaceae were the main denitrifiers in this study, accounting for 65.6-78.2% of the total nirS sequences. Rhodanobacter and unclassified genus of Rhodocyclaceae were significantly (P < 0.05) affected by the sampling time, and 10 genera (Aromatoleum, Bradyrhizobium, unclassified genus of Proteobacteria, unclassified genus of Rhodocyclaceae, unclassified genus of Betaproteobacteria, Dechloromonas, Azoarcus, Thiobacillus, Rubrivivax and Halomonas) were significantly (P < 0.05) affected by land use and fertilization. The PCoA analyses and pairwise tests revealed that the nirS-type denitrifying bacterial communities were affected more by land use than by seasonal change. We also found some OTUs that were detectable in NoF, CF, CFM or NR, and the classification and importance of these OTUs to N cycling merits further investigation.